1. Field of the Invention
This invention relates to refining steel containing residual metals as impurities and to apparatus for this purpose.
2. Description of the Prior Art
Today, steel scrap is widely used as feed stock for steelmaking. Scrap is derived from various sources and, unavoidably, contains other residual metals which, if not removed, reduce the quality of the product steel.
The standard refining processes using oxygen, for example the BOF and the QBOP processes, are effective in removing most of the impurities. However, they do not remove certain residual metals, particularly copper and tin. So steel made from low grade scrap, in which the copper content is above about 0.15% by weight, is poor in hot working properties because the copper goes into a liquid phase at the grain boundaries and cracks occur when the steel is hot worked.
A partial solution to this problem has been to select the scrap carefully from the more expensive grades to match the end use of the steel. So the steelmaker faces the dilemma of paying a higher price for higher grade scrap or of using a lower grade scrap and finding some way of eliminating the impurities.
One suggestion has been to remove metallic impurities from steel by treatment with a halide-containing compound. See, for example, U.S. Pat. No. 3,441,404. This type of process introduces safety and hygiene problems, and insofar as the applicant is aware, has not been used commercially.
There has also been some research done on removing residuals from molten steel by vacuum treatment. See, for example, Salomon-de Frieberg and Davenport, `Vacuum Removal of Copper from Melted Steel Scrap`, The Metallurgical Society of CIM, Annual Volume, 1977, and Harris and Davenport, `Pilot Plant Scale Vacuum Distillation of Liquid Steel to Remove Copper`, Canadian Metallurgical Quarterly, Volume 18, 1979. Both papers show that although copper could be removed from steel by vacuum treatment, the projected times of 90 to 240 minutes were too long for practical purposes. Apart from the cost factor of such a long treatment period, the superheat in the steel would decrease to below its liquidus temperature and make casting impossible. This coupled with inconsistency of the results reported in earlier work led to disenchantment with vacuum refining as a potential commercial means of upgrading steel from melted scrap containing more than tolerable amounts of metallic impurities, particularly copper.
Another use of vacuum is described in U.S. Pat. No. 4,227,922. This process treated solid scrap, e.g. at a temperature less than 1000 K. and indicates that the refining kinetics are slow and unacceptable in tonnage operation. The process does not eliminate tin which is dissolved in the scrap and does not eliminate copper to any great extent at all. There is no suggestion that the process is applicable to molten steel.